Exhaust gases from vehicle engines primarily contain carbon oxides (CO and CO2), nitrogen oxides (NOx), hydrocarbons, sulfur dioxide, and soot. At present, one of the most significant problems is removal of nitrogen oxides, NOx, which are produced during high temperature combustion. In the case of “lean-burn” or partial lean-burn engines, in which there is an excess of oxygen in the exhaust gases, the reduction of NOx to N2 is particularly difficult because reducing components in the exhaust are often completely consumed by the oxygen that is present in large excess.
Catalysts are employed in the exhaust systems of automotive vehicles to convert CO, CO2, hydrocarbons, and NOx, produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric air/fuel ratio, catalysts containing palladium, platinum or rhodium, also known as “three way catalysts,” are able to efficiently convert all the gases simultaneously. However, when the engine is operated in “lean-burn” conditions, to realize a benefit in fuel economy, such three way catalysts are able to convert CO and hydrocarbons, but are not efficient in the reduction of NOx.
Previous attempts to develop a Lean NOx Catalyst (LNC) that will selectively catalyze NOx reduction by HC's has met with limited success. Catalyst materials developed to date that catalyze the HC-NOx reaction allow only about 30 to 50% NOx conversion under conditions of interest. These catalysts are usually either platinum (Pt) group metals (PGM) containing materials that function only at low temperatures (150-200° C.) or base metal materials that function at higher temperatures (300-600° C.). The LNC approach on its own, however, is not sufficient to achieve acceptable NOx reduction to allow future legislated limits to be achieved.
Certain alkali or alkaline earth metals such as potassium or strontium in combination with platinum are capable of storing or adsorbing nitrogen oxides under lean conditions, or in conditions of excess oxygen. More specifically, the platinum first oxidizes NO to NO2 and the NO2 subsequently forms a nitrate complex with the alkali or alkaline earth material. For simplicity herein, this sequence of reactions and adsorption shall be referred to as nitrogen oxides being adsorbed, even though NO is not adsorbed but is actually first converted to NO2 which is then adsorbed. In a rich environment caused, for example, by a regeneration pulse, the nitrate is thermodynamically unstable and the stored NOx is released. The NOx then, with the aid of a catalyst, reacts with reducing species in the exhaust gas to form N2. These adsorbents are known as Lean NOx Trap catalysts (LNT).
Some shortcomings have been identified for the LNT approach. First, a limited operating temperature window exists for the LNT. As with three-way catalysts, a minimum temperature is required for NOx adsorption and conversion. However, unlike the three-way catalysts, NOx adsorption and conversion decreases with increasing temperature above a certain temperature (usually about 350 to 400° C.), due to decreasing stability of the adsorbed nitrate. A second shortcoming of the LNT is the high cost due to the use of platinum group metals.
There remains a need for improved NOx conversion catalysts for automotive lean-burn operation emissions.